Split up the giant switch statement in homectl's parse_argv (#40059)

This is in preparation for later changes. The 1500+ line switch statement was very
hard to work with and had quite a bit of duplicated code. This is a rework that
splits outs parts into functions. I split this up into a bunch of commits so that it
can be reasonably reviewed. When viewed as one giant commit, diff just shows
a giant block of removed code and a giant block of added code.

In some cases I wasn't sure why one or the other of the identity arrays is used…
I preserved existing behaviour in all cases.

test/units/TEST-46-HOMED.sh

@@ -578,8 +578,10 @@ cleanup_ssh() (
     systemctl is-active -q mysshserver.socket && systemctl stop mysshserver.socket
     rm -f /tmp/homed.id_ecdsa /run/systemd/system/mysshserver{@.service,.socket}
     systemctl daemon-reload
-    wait_for_state homedsshtest inactive
-    homectl remove homedsshtest
+    if homectl inspect homedsshtest &>/dev/null; then
+        wait_for_state homedsshtest inactive
+        homectl remove homedsshtest
+    fi
     for dir in /etc /usr/lib; do
         if [[ -f "$dir/pam.d/sshd.bak" ]]; then
             mv "$dir/pam.d/sshd.bak" "$dir/pam.d/sshd"